Fluid ejection devices in inkjet printers provide drop-on-demand ejection of ink droplets. In general, inkjet printers print images by ejecting ink droplets through a plurality of nozzles onto a print medium, such as a sheet of paper. The nozzles are typically arranged in one or more arrays, such that properly sequenced ejection of ink droplets from the nozzles causes characters or other images to be printed on the print medium as the printhead and the print medium move relative to each other. In a specific example, a thermal inkjet printhead ejects droplets from a nozzle by passing electrical current through a heating element in a firing chamber. Heat from the heating element vaporizes a small portion of the fluid in the chamber, and the expanding vapor bubble forces a drop of ink from the chamber through the nozzle. When the heating element cools, the vapor bubble quickly collapses and draws more fluid through fluid feed holes into the chamber to refill the void left by the ejected fluid drop.
During printing, this ejection process can repeat thousands of times per second, and it is therefore important that the heating element be mechanically robust and energy efficient in ejecting droplets. However, there are a number of ways that the heating element can become compromised during printing. For example, the resistive heating element will corrode rapidly and be rendered ineffective if ink contacts the hot, high voltage resistor surface of the heating element. One way that ink comes in contact with the heating element is through the repeated collapsing of vapor bubbles which leads to cavitation damage to the surface material (cavitation layer) that coats the heating element. Each of the millions of collapse events ablates the material in the cavitation layer and ink eventually penetrates through and comes in direct contact with the heating element. Ink can also contact the heating element through chemical erosion or etching away of the passivation layer that underlies the cavitation layer. Wherever the passivation layer is exposed to ink, therefore, chemical etching of the passivation layer can eventually bring ink into direct contact with the heating element.